1. Field of the Invention
The present invention relates to a steering control apparatus for a vehicle such as an automobile.
2. Description of the Related Art
In recent years, a steering control apparatus is being put into practical use which has a function added to improve the steerability or steering stability of a vehicle by correcting an output torque of a motor in an electric power steering apparatus on the basis of a correction signal which is based on a signal from a predetermined vehicle state quantity.
In conventional steering control apparatuses, a redundant system construction adapted to the above-mentioned apparatus is achieved by monitoring steering control in accordance with not only the steering torque but also the correction signal (see, for instance, a first patent document: Japanese patent application laid-open No. 2000-190862).
That is, the conventional apparatus as described in the first patent document includes a motor for applying an assist steering force to a steering system, a torque sensor for detecting the steering torque of the steering system, a steering control section for calculating a target current that is obtained by correcting a target current calculated based on the steering torque detected by the torque sensor on the basis of a correction signal which is based on a signal from a predetermined vehicle state quantity, and a steering control verification section for verifying the presence or absence of abnormality in the steering control section by making a comparison at least between a reference value based on the steering torque and the target current. In addition, two kinds of reference values are set so that when a correction signal is generated, switching is made between the two kinds of reference values.
In the construction of the apparatus described in the above-mentioned first patent document, no reference is made to a method for verifying a motor control section that serves to perform arithmetic calculations for driving the motor based on a target current.
Accordingly, another conventional steering control apparatus has been proposed in which a motor control verification section for verifying an abnormality of a motor control section is provided separately from a steering control verification section (see, for instance, a second patent document: Japanese patent application laid-open No. 2002-67988).
That is, the conventional apparatus described in the second patent document includes a steering control verification section for determining an abnormality of a steering control section by making a comparison between steering torque and a target current, and a motor control verification section for determining an abnormality of a motor control section by making a comparison between a target current and a drive signal supplied to the motor drive circuit. In this case, the motor control verification section monitors a correlation between the direction of the target current and a motor driving direction instruction signal.
For example, in case where an H type bridge circuit is used as the motor drive circuit, the direction of the target current and the motor driving direction instruction signal are in one to one correspondence with each other, so the motor driving direction instruction signal can be easily generated. Accordingly, in the steering control apparatus using a DC motor adapted to be driven by such an H bridge circuit, the motor drive circuit can be monitored by using the conventional apparatus as described in the above-mentioned second patent document.
In conventional steering control apparatuses, when it is difficult to generate a motor driving direction instruction signal from a signal supplied to a motor drive circuit even with the use of the motor control verification section described in the above-mentioned second patent document, such as when a brushless motor is driven by a three-phase inverter, it is necessary to output a motor driving direction instruction signal from a separate microcomputer port for instance, thus resulting in a problem that it becomes difficult to monitor the motor drive circuit.